Current operational HF over-the-horizon (OTH) radars are now accepted as effective and comparatively low-cost wide-area surveillance sensors. They are routinely used to provide air and surface situational awareness over vast regions of land and sea. The technology in, and performance of, these current generation radars is impressive, however, there are many advanced concepts yet to be realized in operational systems. Future OTHR are expected to deliver dramatically improved capability in every performance dimension. While all facets of OTHR systems are being re-considered, in this chapter, we concentrate on one aspect of the transmit sub-system.